The present invention relates to a liquid scintillation spectrometry process and apparatus permitting the analysis of samples having a quenching of the fluorescence efficiency resulting from both a chromatic quenching and a chemical quenching.
It is known that one of the disadvantages of liquid scintillation spectrometry is the quenching of the fluorescence efficiency (number of fluorescence photons emitted per unit of energy dissipated in the scintillator) which generally leads to a reduction in the measuring efficiency (number of pulses counted per unit of time for a given sample). This quenching cannot be calculated because it is dependent on a large number of parameters, many of which are not known at the time of measuring. It is therefore necessary to determine it experimentally for each sample.
A large number of processes have been suggested for this purpose. One for example consists of the use of an external standard radioactive source, whereby a supplementary measurement of the spectrum induced by it is performed in the sample. It is possible to determine the overall quenching of the sample by measuring the relationship of the counting rates in two different energy channels.
Another process is known which uses a standard external radioactive source and which no longer consists of merely measuring the quenching and instead also automatically corrects the sensitivity of the spectrometer in order to compensate this effect. The latter process formed the object of U.S. Pat. No. 3,560,744 granted to T. Jordan on Feb. 2nd 1971 and entitled "Method and apparatus for compensating quenching in liquid scintillation counting", the latter patent being considered as incorporated into the present description.
However, these correction methods are only accurate if the fluorescence quenching is not due both to chemical quenching and chromatic quenching. In this case the fluorescence efficiency of the solution is dependent on the respective contributions of these two effects in such a way that if said contributions are unknown, it is impossible to make a precise correction of the counting rate.
In order to facilitate the description which follows, it is pointed out that the time chemical quenching designates a phenomenon which leads to a reduction in the number of photons emitted by a liquid scintillator under the action of chemical agents, so-called quenchers, present in the solution. It is also pointed out that colour or chromatic quenching is a phenomenon which leads to a reduction in the number of photons reaching the detection means due to their partial absorption on passing through the liquid scintillator.
Thus, chemical quenching is due to a molecular process which occurs at the pont of fluorescent radiation emission, whilst chromatic quenching occurs after said emission.
The necessity of taking separate account of these two types of quenching has already been noted. My prior application Ser. No. 889,885, filed on Mar. 24, 1978, now U.S. Pat. No. 4,187,426 entitled "Liquid scintillation spectrometry process and apparatus" contains a discussion of this question and the latter specification must also be considered as incorporated into the present description. It also describes a liquid scintillation spectrometry process and apparatus taking account of the two types of quenching. To this end, a parameter B is measured which gives the respective contributions of chromatic quenching and chemical quenching. According to said invention, parameter B is obtained in the following way. Through the sample to be measured is passed a scintillation radiation produced by a standard fluorescent source placed in the vicinity of the sample, said source comprising, for example, a solid scintillator exposed to an .alpha. or .beta. radiation source. The scintillation supplied by this standard source supplies an amplitude spectrum which is measured. One of the significant characteristics of the said spectrum is determined, for example the counting rate ratio in two different energy channels. It is this ratio which gives parameter B.
The main parameter G determining the overall quenching of the sample is determined, as in the known methods, by using a standard source .gamma. and then measuring the counting rate ratios in two different energy channels. The knowledge of the two parameters B and G makes it possible to accurately determine the fluorescence efficiency and consequently to correct the spectrometer.
Thus, this procedure requires the use of two external sources, one emitting a fluorescent radiation and the other an ionizing radiation. It is possible to use only one source, naturally provided that it emits both radiation types, but in any case two different spectrometries are required for the two radiation types used. The present invention obviates this problem and proposes a different procedure.